Method for the manufacturing of liquid metal embrittlement resistant zinc coated steel sheet

ABSTRACT

The present invention relates to a method for the manufacture of a coated steel sheet comprising the following successive steps: A. the coating of the steel sheet with a first coating consisting of nickel and having a thickness between 600 nm and 1400 nm, the steel sheet having the following composition in weight: 0.10&lt;C&lt;0.40%, 1.5&lt;Mn&lt;3.0%, 0.7&lt;Si&lt;3.0%, 0.05&lt;Al&lt;1.0%, 0.75&lt;(Si+Al)&lt;3.0%, and on a purely optional basis, one or more elements such as Nb≤0.5%, B≤0.010%, Cr≤1.0%, Mo≤0.50%, Ni≤1.0%, Ti≤0.5%, the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, B. the recrystallization annealing at a temperature between 820 to 1200° C., C. the coating with a second coating based on zinc not comprising nickel.

The present invention relates to a method for the manufacture of a zincbased coated steel sheet. The invention is particularly well suited forthe manufacture of automotive vehicles.

BACKGROUND

Zinc based coatings are generally used because they allow for protectionagainst corrosion, thanks to barrier as well as cathodic protection. Thebarrier effect is obtained by the application of the metallic coating onsteel surface. Thus, the metallic coating prevents the contact betweensteel and corrosive atmosphere. The barrier effect is independent fromthe nature of the coating and the substrate. On the contrary,sacrificial cathodic protection is based on the fact that zinc is ametal less noble than steel. Thus, if corrosion occurs, zinc is consumedpreferentially as compared to steel. Cathodic protection is essential inareas where steel is directly exposed to corrosive atmosphere, like cutedges where surrounding zinc will be consumed before steel.

However, when heating steps are performed on such zinc coated steelsheets, for example hot press hardening, welding, cracks are observed insteel which propagate from the steel/coating interface. Indeed,occasionally, there is a reduction of metal mechanical properties suchas ductility due to the presence of cracks in coated steel sheet afterabove operation. These cracks appear due to following conditions: hightemperature; contact with a liquid metal having a low melting point(such as zinc) in addition to the presence of tensile stress;heterogeneous diffusion of molten metal in substrate grain and grainboundaries. The designation for such phenomenon is known as liquid metalembrittlement (LME), also called liquid metal assisted cracking (LMAC).

The patent application JPS589965 discloses a surface-treated steel sheetobtained by subjecting both surfaces of a steel sheet to electroplatingwith any one of Ni, Cr, Zn, Zn—Ni alloy or Sn—Ni alloy, and heating in anon-oxidizing atmosphere to form a diffusion layer of the plating metalin the substrate steel, and subjecting one surface of the resultingplated steel sheet to hot-dip galvanizing process to form a galvanizedlayer. It is cited that the coating weight of galvanized layer may bereduced, which is extremely advantageous from the viewpoint ofweldability and economic efficiency.

Indeed, above patent application shows the surface-treated steel sheethas an improved weldability only due to the decrease of the zinc coatingweight. Moreover, there is no mention of LME resistance improvement,especially for high strength steels having alloying elements includingMn, Al and Si.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a coated steel sheetwhich shows a high LME resistance behavior. It aims to make available,in particular, an industrially easily implementable method in order toobtain an assembly which improves LME resistance especially after thehot press forming and/or the welding.

The present invention provides a method for the manufacture of a coatedsteel sheet comprising the following successive steps:

-   -   A. the coating of the steel sheet with a first coating        consisting of nickel and having a thickness between 600 nm and        1400 nm, the steel sheet having the following composition in        weight:        -   0.10<C<0.40%,        -   1.5<Mn<3.0%,        -   0.7<Si<3.0%,        -   0.05<Al<1.0%,        -   0.75<(Si+Al)<3.0%,        -   and on a purely optional basis, one or more elements such as        -   Nb≤0.5%,        -   B≤0.010%,        -   Cr≤1.0%,        -   Mo≤0.50%,        -   Ni≤1.0%,        -   Ti≤0.5%,    -   the remainder of the composition making up of iron and        inevitable impurities resulting from the elaboration,    -   B. the recrystallization annealing of said coated steel sheet at        a temperature between 820 to 1200° C.,    -   C. the coating of the steel sheet obtained in step B) with a        second coating based on zinc not comprising nickel.

A coated steel sheet, a spot welded joint and the use of a steel sheetare also provided.

Other characteristics and advantages of the invention will becomeapparent from the following detailed description of the invention.

The designation “steel” or “steel sheet” means a steel sheet, a coil, aplate having a composition allowing the part to achieve a tensilestrength up to 2500 MPa and more preferably up to 2000 MPa. For example,the tensile strength is above or equal to 500 MPa, preferably above orequal to 980 MPa, advantageously above or equal to 1180 MPa and evenabove or equal 1470 MPa.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows schematically a non-limiting example of a spot welded jointwith three coated steel sheets made according to the present invention.

DETAILED DESCRIPTION

The invention relates to method for the manufacturing of a coated steelsheet comprising the following successive steps:

-   -   A. the coating of the steel sheet with a first coating        consisting of nickel and having a thickness between 600 nm and        1400 nm, the steel sheet having the following composition in        weight percent        -   0.10<C<0.40%,        -   1.5<Mn<3.0%,        -   0.7<Si<3.0%,        -   0.05<Al<1.0%,        -   0.75<(Si+Al)<3.0%,        -   and on a purely optional basis, one or more elements such as        -   Nb≤0.5%,        -   B≤0.010%,        -   Cr≤1.0%,        -   Mo≤0.50%,        -   Ni≤1.0%,        -   Ti≤0.5%,    -   the remainder of the composition making up of iron and        inevitable impurities resulting from the elaboration,    -   B. the recrystallization annealing of said coated steel sheet at        a temperature between 820 to 1200° C.,    -   C. the coating of the steel sheet obtained in step B) with a        second coating based on zinc not comprising nickel.

Without willing to be bound by any theory, it seems that in order toobtain a steel sheet having the specific above composition with highresistance to LME, it is an essential feature to deposit the firstcoating of nickel on the sheet steel before the recrystallizationannealing. During recrystallization annealing Ni diffuses towards thesubstrate steel sheet allowing formation of a Fe—Ni alloy layer. Indeed,Ni rich layer concentrates in the surface and sub-surface area of thesteel sheet and thus preventing liquid zinc penetration into the steelduring any heating steps such as welding. Thus, by applying the abovemethod according to the present invention, it is possible to obtain abarrier or buffer layer which prevents LME.

If the first coating consisting of nickel has a thickness below 600 nm,there is a possibility of significant decrease of LME resistancebehavior of the specific above coated steel sheet. Indeed, it seems thatthere is not enough Ni present in the surface and sub-surface region ofsteel sheet which provides enough barrier against LME.

For above steel composition, if the first coating consisting of nickelhaving thickness above 1400 nm, then after recrystallization annealingthe amount of iron in the Fe—Ni alloy layer which is formed in thesub-surface and surface area is very low and is insufficient to forminhibition during subsequent hot dip galvanizing process. Due topresence of higher amount of Ni, a considerable amount of Ni diffuses inthe steel substrate during recrystallization annealing and on the otherhand, due to absence of inhibition layer, Ni also diffuses in thegalvanized coating. Due to presence of higher amount of Ni in thecoating, LME resistance behavior reduces. Moreover, the galvanizedcoating quality is poor due to absence of inhibition layer along withpresence of higher amount of Ni in the coating.

The first coating consists of Nickel, i.e. Ni amount is >99 wt. % and<1% is unavoidable impurities.

The first coating can be deposited by any deposition method known by theperson skilled in the art. It can be deposited by vacuum deposition orelectro-plating or roll coating method. Preferably, it is deposited byelectro-plating method.

Preferably, in step A), the first coating has a thickness between 600and 950 nm. Preferably, in step A), the first coating has a thicknessbetween 600 and 750 nm or between 750 and 950 nm.

Preferably, in step B), the recrystallization annealing is a continuousannealing which comprises continuous pre-heating, heating, soaking andcooling step.

Advantageously, the recrystallization annealing is performed in anatmosphere comprising from 1 to 30% of H₂ at a dew point between −60 and+30° C. or a dew point below 60° C. For example, the atmospherecomprises from 1 to 10% of H₂ at a dew point between −60° C. and −30° C.In another embodiment, the recrystallization annealing is performed from1 to 30% of H₂ at a dew point between −30 and +30° C. Preferably, therecrystallization annealing is performed from 1 to 30% of H₂ at a dewpoint between −10 and +10° C. Indeed, without willing to be bound by anytheory, it is believed that this dew point further improves thecoatability of the steel sheet according to the present inventionwithout considerable decrease of any mechanical property.

Advantageously, in step C), the second layer comprises above 50%, morepreferably above 75% of zinc and advantageously above 90% of zinc. Thesecond layer can be deposited by any deposition method known by the manskilled in the art. It can be by hot-dip coating, by vacuum depositionor by electro-galvanizing.

For example, the coating based on zinc comprises from 0.01 to 8.0% Al,optionally 0.2-8.0% Mg, the remainder being Zn.

Preferably, the coating based on zinc is deposited by hot-dipgalvanizing method. In this embodiment, the molten bath can alsocomprise unavoidable impurities and residuals elements from feedingingots or from the passage of the steel sheet in the molten bath. Forexample, the optionally impurities are chosen from Sr, Sb, Pb, Ti, Ca,Mn, Sn, La, Ce, Cr, Zr or Bi, the content by weight of each additionalelement being inferior to 0.3% by weight. The residual elements fromfeeding ingots or from the passage of the steel sheet in the molten bathcan be iron with a content up to 5.0%, preferably 3.0% by weight.

In a preferred embodiment, the second layer consists of zinc. When thecoating is deposited by hot-dip galvanizing process, the percentage ofAl is comprised between 0.15 and 0.40 wt. % in the bath. Moreover, theiron presents in the first coating after recrystallization annealingreacts with aluminum and forms the inhibition layer. Thus, it providesreactive wetting behavior during hot dip galvanizing.

With the method according to the present invention, a steel sheet coatedwith a diffused alloy layer comprising iron and nickel formed throughdiffusion of nickel into the steel, such layer being directly topped bya zinc based layer is obtained. It is believed that the diffused alloylayer acts like a barrier layer against LME.

Preferably, the steel sheet has a microstructure comprising from 1 to50% of residual austenite, from 1 to 60% of martensite and optionally atleast one element chosen from: bainite, ferrite, cementite and pearlite.In this case, the martensite can be tempered or untempered.

In a preferred embodiment, the steel sheet has a microstructurecomprising from 5 to 45% of residual austenite.

Preferably, the steel sheet has a microstructure comprising from 1 to60% and more preferably between 10 to 60% of tempered martensite.

Advantageously, the steel sheet has a microstructure comprising from 10to 40% of bainite, such bainite comprising from 10 to 20% of lowerbainite, from 0 to 15% of upper bainite and from 0 to 5% of carbide freebainite.

Preferably, the steel sheet has a microstructure comprising from 1 to25% of ferrite.

Preferably, the steel sheet has a microstructure comprising from 1 to15% untempered martensite.

Advantageously, the steel sheet has a decarburized layer having a depthmaximum of 40 μm, preferably maximum of 30 μm and more preferablymaximum of 20 μm on either side of the sub-surface area. Thedecarburization is defined in the norm ISO 3887:2017. Indeed, withoutwilling to be bound by any theory, it is believed that the decarburizedlayer further improves the LME resistance without considerablydecreasing the mechanical properties of the steel sheet.

Preferably, an internal oxides layer having a thickness below or equalto 5 μm is present in the steel sheet. Without willing to be bound byany theory, it is believed that this layer leads to a good coatabilityof the zinc coating since the continuous inhibition layer Fe₂Al₅ isformed which represents good reactive wetting.

After the manufacture of a steel sheet, in order to produce some partsof a vehicle, it is known to assembly by welding two or more metalsheets. Thus, a spot welded joint is formed during the welding of atleast two metal sheets, said spot being the link between the at leasttwo metal sheets.

To produce a spot welded joint according to the invention, the weldingis performed with an effective welding current between 3 kA and 15 kAand the force applied on the electrodes is between 150 and 850 daN withsaid electrode active face diameter being between 4 and 10 mm.

Thus, a spot-welded joint of at least two metal sheets comprising atleast a steel sheet, comprising the coated steel sheet according to thepresent invention, is obtained. The above said joint contains less than2 cracks having a size above 100 μm and wherein the longest crack has alength below 450 μm.

Preferably, the second metal sheet is a steel sheet or an aluminumsheet. More preferably, the second metal sheet is a steel sheetaccording to the present invention.

In another embodiment, the spot welded joint comprises a third metalsheet being a steel sheet or an aluminum sheet. For example, the thirdmetal sheet is a steel sheet according to the present invention. FIG. 1thus shows schematically a spot weld 40 for joining three coated metalsheets each with a steel sheet 10, 20, 30, first coating 12, 22, 32 andsecond coating 14, 24, 34, respectively.

The steel sheet or the spot welded joint according to the presentinvention can be used for the manufacture of parts for automotivevehicle.

The invention will now be explained in trials carried out forinformation only. They are not limiting.

EXAMPLES Example 1: Optimization of Ni Coating Thickness with Respect toLME Resistance Behavior

For all samples, steel sheets used have the following composition inweight percent: C=0.37%, Mn=1.9%, Si=1.9%, Cr=0.35%, Al=0.05% andMo=0.1%.

In Trial 1, steel was annealed in an atmosphere comprising 5% of H₂ and95% of N₂ at a dew point of −45° C. The annealing was carried out at900° C. for 132 seconds. After that steel was quenched at 210° C.followed by partitioned at 410° C. for 88 seconds. Finally, the steelsheet was cooled to room temperature. On the annealed steel sheet, aZinc coating was applied by an electro-galvanizing method.

In Trials 2 to 6, Ni was first deposited by an electro-plating method tohave a thickness of 150, 400, 650, 900 nm and 1600 nm respectively onfull hard steel sheets before annealing. After that, the pre-coatedsteel sheets were annealed in an atmosphere comprising 5% of H₂ and 95%of N₂ at a dew point of −45° C. The annealing was carried out at 900° C.for 132 seconds. At the end of the annealing, the steel sheets werecooled to a quench temperature of 210° C. and again heated at apartitioning temperature of 410° C. Partitioning was carried out for 88s and then again heated up to a galvanizing temperature of 460° C. and aZinc coating was applied by hot dip coating method using a liquid Zincbath containing 0.20 wt. % Al maintained at 460° C. The objective ofabove trials was to determine the optimum Ni coating thickness whichprovides excellent LME resistance behavior. The susceptibility of LME ofabove coated steel was evaluated by resistance spot welding method. Tothis end, for each Trial, three coated steel sheets were welded togetherby resistance spot welding. The type of the electrode was ISO Type Bwith a face diameter of 6 mm; the force of the electrode was of 5 kN andthe flow rate of water of was 1.5 g/min. the welding cycle was reportedin Table 1:

TABLE 1 Welding schedule to determine optimum Ni coating thickness Weldtime Pulses Pulse (cy) Cool time (cy) Hold time (cy) Cycle 2 12 2 15

The LME crack resistance behavior was evaluated using 3 layer stack-upconditions. The number of cracks having crack length of more than 100 μmwas then evaluated using an optical microscope as reported in Table 2.

TABLE 2 LME crack details after spot welding (3 layer stack-upconditions) for Trials 1 to 6. Number of Maximum Dew cracks per crackPoint 2^(nd) spot weld length Trials (° C.) 1^(st) coating coating (>100μm) (μm) Trial 1 −45° C. — Zn (EG) 7 850 Trial 2 −45° C. Ni (150 nm) Zn(GI) 3 620 Trial 3 −45° C. Ni (400 nm) Zn (GI) 2 500  Trial 4* −45° C.Ni (650 nm) Zn (GI) 2 420  Trial 5* −45° C. Ni (900 nm) Zn (GI) 1 420Trial 6 −45° C.  Ni (1600 nm) Zn (GI) 1 680 *according to the presentinvention.

Trials 4 and 5 according to the present invention show an excellentresistance to LME as compared to Trials 1, 2, 3 and 6. Indeed, thenumber of cracks above 100 μm is below or equal to 2 and the longestcrack has a length below 450 μm. It results in a reduction of the amountof heat input during spot welding and thus causes a significantreduction of number of cracks formation due to LME.

The LME crack resistance behavior was also evaluated using 2 layerstack-up conditions for Trials 1, 4 and 5. In this condition, two coatedsteel sheets were welded together by resistance spot welding. The numberof cracks above 100 μm was then evaluated using an optical microscope asreported in Table 3.

TABLE 3 LME crack details after spot welding (2 layer stack-upconditions) for Trials 1, 4 and 5. Number of cracks per Maximum crackTrials spot weld (>100 μm) length (μm) Trial 1  3 750 Trial 4* 1 170Trial 5* 1 300 *according to the present invention.

Trials 4 and 5 according to the present invention show an excellentresistance to LME as compared to Trial 1. Indeed, the number of cracksabove 100 μm is of 1 and the longest crack has a length of 300 μm. Itresults in a reduction of the amount of heat input during spot weldingand thus causes a significant reduction of number of cracks formationdue to LME.

From above trial, excellent LME resistance behavior was observed when Nicoating thickness was maintained between 600 to 1400 nm. In order toenhance the LME resistance further, sub-surface area of the steel sheetwas modified by formation of decarburized layer. Example 2 representsthe combined effect of decarburized layer along with Ni coating having aspecific thickness.

Example 2: Effect of Decarburization of Steel Sub-Surface Along with NiCoating on LME Resistance Behavior

In order to prevent any decarburization, in Trial 7, steel was annealedin an atmosphere comprising 5% of H₂ and 95% of N₂ at a dew point of−80° C. The annealing was carried out at 900° C. for 132 seconds. Afterthat steel was quenched at 210° C. followed by partitioned at 410° C.for 88 seconds. Finally, the steel sheet was cooled to room temperature.On the annealed steel sheet, a Zinc coating was applied by theelectro-galvanizing method.

In Trials 8 and 9, Ni was first deposited by the electro-plating methodto have a thickness of 900 nm on full hard steel sheets beforeannealing. After that, the pre-coated steel sheets were annealed in anatmosphere comprising 5% of H₂ and 95% of N₂ at a dew point of −80° C.,for trial 8, without any decarburized layer in the sub-surface area ofthe steel. For trial 9, the annealing dew point was maintained at −10°C. with 5% of H₂ and 95% of N₂. For trial 8 and 9, the annealing wascarried out at 900° C. for 132 seconds. At the end of the annealing, thesteel sheets were cooled to quench temperature of 210° C. and againheated at partitioning temperature of 410° C. Partitioning was carriedout for 88 s. Finally, the steel sheet was cooled to room temperature.On the annealed steel sheet, the Zinc coating was applied by theelectro-galvanizing method.

Table 4 compares the decarburized layer thickness when the steel wasannealed at different dew point without and with Ni coating. Withoutcompromising steel mechanical properties, the decarburized layerthickness was restricted by controlling annealing dew point.

TABLE 4 Decarburized layer thickness of the sub-surface area of thesteel sheet after annealing at different dew point Decarburized TrialsDP (° C.) 1^(st) coating 2^(nd) coating layer (μm) Trial 7  −80 — Zn(EG) 0 Trial 8* −80 Ni (900 nm) Zn (EG) 0 Trial 9* −10 Ni (900 nm) Zn(EG) 15 *according to the present invention.

The LME susceptibility of above coated steels (Trials 7, 8 and 9) wasevaluated by resistance spot welding method. For this purpose, for eachTrial, three coated steel sheets were welded together by resistance spotwelding. The type of the electrode was ISO Type B with a face diameterof 6 mm; the force of the electrode was of 5 kN and the flow rate ofwater of was 1.5 g/min. The welding cycle was reported in Table 5:

TABLE 5 Welding schedule, to determine combined effect of Ni coating anddecarburized layer Weld time Pulses Pulse (cy) Cool time (cy) Hold time(cy) Cycle 1 23 NA 18

The LME crack resistance behavior was evaluated using 2 layer stack-upconditions for Trials 7, 8 and 9. In this condition, two coated steelsheets were welded together by resistance spot welding. The number ofcracks above 100 μm was then evaluated using an optical microscope asreported in Table 6.

TABLE 6 LME crack details after spot welding (2 layer stack-upconditions) for Trials 7, 8 and 9 Number of cracks average of sum oftotal crack per spot size length above 100 μm per Trials weld (>100 μm)spot-weld (μm) Trial 7  3 573 Trial 8* 1 122 Trial 9* 0 0 *according tothe present invention.

Trials 8 and 9 according to the present invention show a high resistanceto LME as compared to Trial 7. Moreover, For Trial 9, excellent LMEresistance behavior was observed in steel sheet due to combined effectof decarburized layer with Ni layer having a specific thickness.

What is claimed is:
 1. A method for the manufacture of a coated steelsheet, the method comprising the following successive steps: coating ofa steel sheet with a first coating consisting of nickel and having athickness between 600 nm and 950 nm, the steel sheet having thefollowing composition in weight:0.10<C<0.40%,1.5<Mn<3.0%,0.7<Si<3.0%,0.05≤Al<1.0%,0.75<(Si+Al)<3.0%, and on a purely optional basis, one or more elementsincluding:Nb≤0.5%,B≤0.010%,Cr≤1.0%,Mo≤0.50%,Ni≤1.0%,Ti≤0.5%, a remainder of the composition making up of iron and inevitableimpurities resulting from processing; recrystallization annealing of thecoated steel sheet at a temperature between 820 to 1200° C., performedin an atmosphere comprising from 1 to 10% of H₂ at a dew point between−30 and +30° C.; and coating the steel sheet obtained inrecrystallization annealing with a second coating based on zinc notcomprising nickel.
 2. The method as recited in claim 1 wherein the firstcoating has a thickness between 600 and 750 nm.
 3. The method as recitedin claim 1 wherein the recrystallization annealing is a continuousannealing.
 4. The method as recited in claim 1 wherein in therecrystallization annealing is performed in an atmosphere comprisingfrom 1 to 10% of H₂ at a dew point between −10 and +10° C.
 5. The methodas recited in claim 1 wherein the second coating includes above 50% ofzinc.
 6. The method as recited in claim 5 wherein the second coatingincludes above 75% of zinc.
 7. The method as recited in claim 6 whereinthe second coating includes above 90% of zinc.
 8. The method as recitedin claim 7 wherein the second coating consists of zinc.
 9. The method asrecited in claim 1 wherein the first coating has a thickness between 750and 950 nm.
 10. The method as recited in claim 1 wherein the firstcoating has a thickness between 650 and 900 nm.